Monday, June 22, 2009

measuring Porrima (Blue Mountains)

With Dietmar's assistance, with the RASC Toronto Centre's C14, I tried to measure the separation and position angle of Porrima.

While I had forgotten to bring along Teague's article, describing easy-to-do methods which would yield more accurate results, I had a freshly downloaded copy of the Celestron documentation for the Micro Guide eyepiece. It would have to do.

I installed the society's 2" Tele Vue 4x Powermate onto the big SCT. Wow. The view was unusable. So, I grabbed my 1¼" Celestron 2x Barlow from the THO.

Now I needed to measure the drift time given the optical path...
  1. Celestron 14" SCT (on Paramount ME)
  2. Optec Temperature Compensated Focuser (on)
  3. Tele Vue 2" mirror
  4. 2"-to-1¼" adapter
  5. Celestron Ultima 2x Barlow (mine)
  6. Celestron 12.5mm Micro Guide eyepiece (Geoff's)
For some reason, the 2"-to-1¼" adapter did not tightly grip the Barlow. So I had to angle it up a bit to not fall out. And be careful when turning it... It worked out OK.

First, I needed to figure out how to stop tracking with the Paramount. I asked Dietmar but he wasn't entirely sure. So I explored a bit in TheSky6, checking the on-board help and some web sites online. Found something that sounded promising. Tried it and it worked:
  1. Choose the Telescope command.
  2. Hover on the Options command. Tracking is listed in the submenu and checked if on.
  3. Click the Tracking command to disable (or enable).
I roughly aligned the Micro Guide-Barlow assembly to the celestial equator. I put the target star at one end of the Linear Scale. The Celestron documentation said to start from the middle but I did not see the point in that. Then I asked Dietmar to shut off the tracking. After a couple of tries, I had the reticule well aligned.

We repeated this process now to capture the times for the target star to travel across the whole length of the Linear Scale. Using my palmtop Stopwatch program with the lapping feature, I saved the times into a text file. It moved fast!


The first time I started with the star on the 0 (zero) mark, told Dietmar to "go," and then when I saw the field start to move, I started the timer. I realised this would have inherit delay. For the 2nd and subsequent tries, I put the target star outside the Large Circular Scale, and then hit the lap time key for when it crossed 0 and then 60. Just like a race car.

The mean with all four times is 8.94; the last three times: 9.07.

This gives a scale division of 2.24 to 2.27.

I estimated the primary and secondary stars of Porrima to be approximately 2/3 of the distance between the divisions along the Linear Scale.

I turned the Micro Guide 90° to double-check. The stars straddled just outside the parallel lines of the Linear Scale. I recalled that the parallel lines are half the distance between the tick marks.

Using my scale division values above, this gives a separation of 1.49 to 1.52.

position angle
Using the outer values of the Large Circular Scale, I estimated the position angle to be 30°.


The PA looks way off. But I need to find some "current" PA and Sep figures to know if I'm on or off track. Haas stated that Porrima is PA 179° and sep 0.4" in 2004. And, again, Eric stated 1.25" a couple of weeks back... He did not quote his source. And he did not quote a PA.

Also, interesting to note: after re-reading Teague's article, I learned/realised that it is not useable, his "better way," for extremely tight doubles, when they are not separated by more than 1 tick... Maybe I should have left the 4x TV in...

No worries. I'm just getting my legs. Learning all the methods. And presumably I'll get so comfortable with all of them that I can dash them off and compare the results.

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